Glider

The gauge of the wire depends on what you want to bend. The thicker the twig or branch, the thicker the wire will need to be to hold it.

The way I decide which gauge to use is to take a length of wire as long as the branch I want to bend, and push the end of the wire down onto the end of the branch. If the wire bends before the branch does, the wire is too thin. If the branch bends a bit before the wire does (i.e. if I can get the branch to bow gently before the wire bends), it's right.

The choice between copper or aluminium is entirely up to you. Having said that, there are a few trees on which you should avoid using copper (prunus for example).

Copper wire is stronger by diameter than aluminium, but in thicker gauges, this can make it a lot harder to work with. Aluminium is softer, so you need a thicker gauge to perform the same function as a thinner gauge of copper would.

However, i find that an advantage as a thicker gauge wire uses a greater surface area to apply the same force than a thinner but stronger wire would, which is gentler on trees with delicate bark like azaleas (in fact aluminium wire was invented specificaly for use with azaleas).

For my own wiring, I find aluminium to be easier to use and gentler on the trees than copper so for general wiring I use only aluminium. Having said that, for very fine wiring I use copper (0.5mm) as at this gauge, copper is still quite strong but aluminium is a bit useless. The thinnest aluminium gauge that is useful is 1mm.

Both aluminium and copper work harden and can be annealed to re-soften them (aluminium at a much lower temperature), so the choice is yours.

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